Acidization of carbonate rocks is a common practice to reduce formation damage near the wellbore. In this process, an acidic solution is injected to dissolve some of the rock, creating conductive channels called wormholes. These wormholes facilitate the flow of hydrocarbons to the wellbore. In the literature, there are several theoretical and experimental studies performed to understand this process. Recent work by Maheshwari et al. 2013 focused on a qualitative comparison of 3D numerical results for slow-reacting acids with experimental data and presented a sensitivity analysis of the acidization process with respect to various transport, reaction, and rock properties. There are very few 3D numerical studies that can predict the experimental results quantitatively, such as the optimum injection rate for fast-reacting acids such as hydrochloric acid (HCl). Therefore, the main objective of this study is to quantitatively predict the experimentally observed acidization curve and dissolution patterns in carbonates with HCl.

We present 3D numerical simulations of carbonate acidization with HCl using a two-scale continuum (TSC) model. The model describes the reactive transport at Darcy scale and retains all pore-scale physics through structure-property relations. Unlike previous studies, we use a new two-parameter (pore-broadening and pore-connectivity) structure-property relation to describe the change in permeability, pore radius, and interfacial area per unit volume with porosity. We predict quantitatively the experimentally observed acidization curve for an HCl/limestone system and show the existence of a critical heterogeneity (that corresponds to the minimum amount of acid required to breakthrough). We also present scaling criteria to estimate the wormhole-tip diameter and optimum acid-injection rate for vuggy and nonvuggy carbonates. Finally, we present the flow dynamics of acid inside the wormhole.

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